1. Field of the Invention
The present invention relates to a liquid crystal display device of the simple matrix type.
2. Description of Prior Art
FIG. 14 is a partial cross section of a prior art liquid crystal display device of the simple matrix type. This display device is a color liquid crystal display device of the super twisted nematic type, and comprises a first electrode substrate 10, a second electrode substrate 20, and a liquid crystal layer 9 filled between the first and the second electrode substrates 10 and 20. On a first substrate 1 constituting the first electrode substrate 10 are formed color filters 3 of red (R), green (G), and blue (B). The respective color filters 3 are formed with striping unidirectionally in parallel with one another as shown in FIG. 3. FIG. 14 is a cross section of the display device taken along the line 3--3 in FIG. 3. Between the respective color filters 3 are provided black masks made of material having a low transmissivity in parallel with one another. An insulating film 4 is formed entirely over the color filters 3 and the black masks 11.
On the insulating film 4 are formed first transparent display electrodes 3 in parallel with one another in a direction perpendicular to an extending direction of the color filters 3 as shown in FIG. 2. FIG. 14 is a cross section of the display device taken along the line 2--2 in FIG. 2. An orientation film 6 for regulating an orientation of liquid crystal molecules is formed entirely over the first display electrodes 5.
On a second substrate 2 constituting the second electrode substrate 20 are formed second transparent display electrodes 7 extending in a direction perpendicular to an extending direction of the first display electrodes 5. As shown in FIG. 15, the second display electrodes 7 are formed in parallel with the color filters 3 and overlapping therewith. FIG. 4 is a cross section of the display device taken along the line 5--5 of FIG. 15. An orientation film 8 is formed entirely over the second display electrodes 7.
A color liquid crystal display device comprises the display device shown in FIG. 14 as a drive cell and is generally provided with a compensation cell. A cell structure in the case where the compensation cell is provided is shown in FIG. 17. FIG. 17 shows the display device viewed from the compensation cell. An arrow 32 shows a direction along which an orientation film for an upper substrate of the compensation cell is rubbed. An arrow 33 shows a direction along which an orientation film for a lower substrate of the compensation cell is rubbed. An arrow 34 shows a direction along which an orientation film for an upper substrate of the drive cell is rubbed. An arrow 35 shows a direction along which an orientation film for a lower substrate of the drive cell is rubbed. In whichever the drive cell or the compensation cell, a twist angle of the liquid crystal molecule is set at 240.degree.. A ratio of retardation in the drive cell (a product of anisotropy in index of refraction .DELTA.nc and cell thickness dc) to retardation in the compensation cell (a product of anisotropy in index of refraction .DELTA.nd and cell thickness dd) is set at (.DELTA.nc.times.dc)/(.DELTA.nd.times.dd)=0.90. A solid line 30 shows a direction of a polarization axis of a polarizing plate on the compensation cell side. A broken line 31 shows a direction of a polarization axis of the polarizing plate on the drive cell side. By setting the polarizing plate as above, a display device capable of carrying out a normally black method can be obtained, according to which method a black display is performed when an OFF voltage V-off is applied, while a white display is performed when an ON voltage V-on is applied.
FIG. 16 graphically shows relationship between amplitude of the voltage applied to the liquid crystal layer between the first display electrodes 5 and the second display electrodes 7, and display luminance in the liquid crystal display device of the simple matrix type thus constructed, wherein a vertical axis represents relative display luminance and a horizontal axis represents the amplitude of an operating voltage. In FIG. 16, it is assumed that the display luminance is 100 in the case where a voltage V-on is applied to the liquid crystal layer between the first display electrodes 5 and the second display electrodes 7 when the display device is driven at a duty of 1/240. Then, the luminance is 1.5 in the case where the voltage V-off is applied. Accordingly, in a pixel region formed at an intersection of the first and the second display electrodes 5 and 7, a contrast ratio is greater than 50.
However, in a display device shown in FIG. 14, the black masks 11 formed on the first substrate 1 does not entirely cover the space defined between the first display electrodes 5 as will be seen from FIGS. 2, 3, and 15. Also, the voltage is not to be applied to the liquid crystal layer between the first display electrodes 5. More specifically, since the applied voltage becomes OV, the luminance becomes about 20 as shown in FIG. 16. Accordingly, the contrast ratio of the display screen including the pixel regions and other regions becomes as low as about 8. Low contrast ratio results in degradation of the quality of the display. Further, in a color liquid crystal display device, low contrast ratio causes color purity to exceedingly deteriorate.
Further, in the display device adopting the normally white method for performing the white display when the voltage V-off is applied and the black display when the voltage V-on is applied, the contrast ratio in the regions other than the pixel regions are greatly reduced. Accordingly, degradation of the quality of display and the color purity becomes a further bigger problem for the display device of this type.
In order to prevent the reduction in contrast ratio, it can be considered to form masking films to cover the regions between the first display electrodes. In the case that such masking films are provided on the first electrode substrate 10 having the color filters 3 formed thereon, it is required that the masking films be formed between the first display electrodes and the first substrate 1. Accordingly, the first display electrodes 5 are formed on an uneven surface created by forming the masking films. When the first display electrodes 5 are formed on the uneven surface, the first display electrodes 5 are liable to be disconnected. Further, in the case where a leakage of the display electrodes 5 occurs, it is difficult to correct it. Even in the case where the masking films are formed between the second substrate 2, not having the color filters 3 formed thereon, and the second display electrodes 7, the second display electrodes 7 are liable to be disconnected due to the uneven surface caused by providing the masking films on the second substrate 2.
While more enlarged and highly complex display screens are being manufactured, there is a tendency toward manufacturing more thin and elongated display electrodes. In the color display device, since many pixels corresponding to the respective colors are provided, further thinner display electrodes are required. As the display electrodes become thinner in this way, resistance of the display electrodes increases, which in turn increases burdens on drivers or the like parts. This presents another problem.